Powder coating compositions have become increasingly important because they give off very little or no volatile material to the environment when cured. Typically, any such emissions are limited to by-products of the curing reaction, such as blocking agents or volatile condensation products. Powder coatings have found use as both decorative coatings and protective coatings.
Automotive bodies are generally first coated by the cathodic electrocoat process with an acid-salted, amine-containing polymer. The amine-containing polymer is typically crosslinked with a blocked isocyanate crosslinker. A second primer, often called a primer-surfacer, is applied over the electrocoat layer for additional protection, particularly against corrosion and gravel chipping, and to provide a smoother surface upon which to apply the topcoat. Topcoats for automotive and other industrial applications may be a one-layer coating, in which the color is generally uniform through the coating layer, or a clearcoat-basecoat composite coating, having a colored basecoat layer underlying a transparent clearcoat layer. Basecoat-clearcoat composite coatings are widely used and are notable for desirable gloss, depth of color, distinctness of image and/or special metallic effects. Composite coatings are particularly utilized by the automotive industry to achieve a mirror-like, glossy finish.
It is also important for topcoats, including the clearcoat-basecoat composite coatings, to provide the desired color. When electrocoat primers are used, amine from electrocoat primer layer may migrate during thermal cure of later applied coating layers causing undesirable yellowing in those layers. The migration and yellow occur particularly at higher temperatures.
Ruth et al., U.S. Pat. No. 6,140,430 describes an epoxy powder coating containing a non-crystalline epoxy resin, a small amount of a crystalline epoxy agent, a curing agent reactive with epoxy groups, and a catalyst. The crystalline epoxy resin has a melting point preferably greater than 90° C. It is included in the powder coating in an amount sufficient to reduce the coating viscosity during curing so that air entrapped in the powder particles, which would otherwise become bubbles and causing haze in the coating, can escape.
Sorokin et al., RU 604853, discloses epoxyisocyanurate oligomers for making heat-resistant polymers. The epoxyisocyanurate oligomer is dissolved, for example in cyclohexane, and reacted with anhydride- or acid-functional hardeners. A varnish coating applied to metal or glass reportedly has improved heat resistance, impact strength, and tensile strength. The Sorokin patent, however, does not mention powder coatings or address the problems of powder coatings for automotive application or powder coatings used in composite coatings, particularly regarding cure response and yellowing resistance.